doc/sphinx: Pass right filename as source

With this error output becomes almost readable. The line numbers are
still totally bonghits, but that's a lot harder to pull out of
kerneldoc. We'd essentially have to insert some special markers in the
kernel-doc output, split the output along these markers and then
insert each block separately using

     state_machine.insert_input(block, source, first_line)

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: linux-doc@vger.kernel.org
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

Documentation/sphinx/kernel-doc.py

@@ -57,7 +57,7 @@ class KernelDocDirective(Directive):
         env.note_dependency(os.path.abspath(filename))
 
         tab_width = self.options.get('tab-width', self.state.document.settings.tab_width)
-        source = self.state_machine.input_lines.source(self.lineno - self.state_machine.input_offset - 1)
+        source = filename
 
         # FIXME: make this nicer and more robust against errors
         if 'export' in self.options: